What happened
During arrival procedures at Vancouver International Airport, a Horizon Air de Havillost DHC-8 was on an instrument landing system approach to Runway 08 left, following an Air Canada Airbus A319. The arrival low controller had previously instructed the DHC-8 to maintain a speed of 170 knots until reaching the DAWG fix.
As the DHC-8 approached the fix, the pilot slowed the aircraft to 110 knots without notifying air traffic control. Simultaneously, the controller observed the speed of the DHC-8 decreasing on his display, prompting him to instruct the following Airbus A319 to reduce its speed to 160 knots. Although the Airbus A319 pilot acknowledged this instruction, a subsequent instruction to reduce speed even further was not received because the crew had already switched to the tower frequency as previously directed.
As the separation between the two aircraft eroded, the arrival high controller recognized the potential conflict and contacted the tower to request a missed approach for the Airbus A319. By the time the overshoot maneuver began, the longitudinal spacing between the aircraft had decreased to 1.9 nm, falling below the required 2.5 nm separation standard. There was no risk of collision.
The investigation
The investigation examined the performance of the trainee arrival low controller and the on-the-job instructor (OJI). It was noted that the controller had transferred communications for both aircraft to the tower before relinquishing responsibility for them, which prevented timely instructions.
Technical analysis of the radar data-processing system revealed that the system's smoothing parameters can cause a lag in displaying rapid deceleration. This likely led the controller to underestimate how quickly the DHC-8 was losing speed. Furthermore, the investigation looked into the training environment, noting that the use of minimum separation standards during on-the-job training leaves almost no margin for error when unexpected pilot actions occur.